Disable enable_metadata_network feature in dhcp agent.

This feature is needed only when users may want to advertise 169.254.0.0/16
network from it's local dhcp server and this subnet will be added to
quantum configuration (quantum subnet-create).
Merge pull request #83 from tpaszkowski/pull-req-fix-0-0-in-dhcp-agent

Disable enable_metadata_network feature in dhcp agent.